This invention relates in an improvement in a radio frequency (RF) magnetic field waveform in a nuclear magnetic resonance (NMR) imaging apparatus. More in particular, it relates to a double-slice imaging method for imaging slices double in number in comparison to an ordinary method.
The following references (1) to (3) are known as related art.
(1) H. P. Hafner, "Magnetic Resonance in Medicine", Vol. 13 (1990), pp. 279-292 PA0 (2) G. H. Glover, "POMP Imaging: A New High Efficiency Technique", Society of Magnetic Resonance in Medicine, Seventh Annual Meeting and Exhibition, Vol. 1 (1988), p. 241 PA0 (3) A. A. Maudsly, "Journal of Magnetic Resonance", Vol. 41, (1980), pp. 112-126
The techniques (1) to (3) are accomplished by an NMR imaging apparatus having an RF magnetic field generator having a QD signal generation function shown in FIG. 5. The reference (3) describes the structure of the RF magnetic field generator for generating the QD signal. To generate the QD signal, a radio frequency reference frequency signal is separated into reference frequency signals having four phases of 0.degree., 90.degree., 180.degree. and 270.degree.. After these signals are independently amplitude-modulated, they are again synthesized with one another. In this instance, the reference frequency signals having the phases of 0.degree. and 180.degree. are allowed to correspond to REAL and those having the phases of 90.degree. and 270.degree., to IMAG components. When these REAL and IMAG are independently amplitude-modulated, a plurality of slices can be simultaneously excited with arbitrary weighting.
The reference (1) describes the principle of double-slice imaging and its simulation result. In the case of double-slice imaging for obtaining two separate slice images from two kinds of measured signal data, two slices are simultaneously excited. The absolute values of two signals generated from the two slices after excitation are hereby called S1 and S2. In the first measurement, the reference frequency signals corresponding to REAL and IMAG are amplitude-modulated so as to attain S1+S2, and in the second measurement, to attain S1-S2. S1 can be obtained by (A+B)/2 and S2, by (A -B)/2. In the present invention, however, symbols different from those described in the reference (1) are defined.
The reference (2) describes a method which generally carries out N-fold slice imaging. This reference elaborates so that the calculation for separating the slices proves to be similar to the Fourier transform as the calculation method for image reconstruction. When N=2, however, this reference becomes exactly the same as double-slice imaging described in the reference (1).
The reference (3) describes a definite application by an imaging method which is referred to as "line scanning". This reference makes the most of a spin echo method, and simultaneously excites a plurality of lines on one slice by a weight of an Hadamard matrix. The signals so generated are measured, and each line is separated by subsequent processing. The reference describes a definite application by line scanning imaging and the concept of this reference (3) can be applied to double-slice imaging, too. In other words, double-slice imaging can be carried out when the number of lines is 2 and each line is replaced by a slice, by methods known at the present time.
The effect brought forth by double-slice imaging and N-fold slice imaging lies in that higher efficiency can be obtained by simultaneously exciting a plurality of slices and observing them than when separate excitation and observation is made, as described in the references (2) and (3). When the same number of slices are measured for the same measurement time, a signal-to-noise ratio (S/N) of the final image is higher by simultaneous excitation and observation than by separate excitation and observation.
However, the references (1), (2) and (3) described above cannot be accomplished by the NMR imaging apparatus having the RF magnetic field generator shown in FIG. 3.